1. Field of the Invention
The present invention relates to methods and systems for medical imaging of parts of a patient in which acquisition of data by a medical imaging apparatus for image reconstruction is gated by a combination of electrocardiogram (ECG) and peripheral pulse (PPU) signals from the patient.
2. Description of the Related Art
Many organs and regions of the body of a patient are affected by cardiac and respiratory motions, including, for example, not only the heart, the lungs, and vessels throughout the body, but also abdominal organs, especially upper abdominal organs, and intracranial structures. For the medical imaging of such affected organs, it is often useful, or even necessary, to take these motions into account. On the one hand, in the case of those imaging modalities, such as a conventional x-ray imaging, in which a single image is acquired in a short exposure (for example, a few milliseconds (msec) or less), it is often useful for images to be acquired at known and pre-determined phases of cardiac or respiratory motions.
However, on the other hand, in the case of those imaging modalities which reconstruct a recognizable image from imaging data acquired over a period of time (for example, 100 msec. or more), it is often necessary to take body motion into account in order to avoid motion artifacts that degrade reconstructed images and render them less useful clinically. Currently, important modalities of this nature include magnetic resonance (MR) imaging, computer tomographic (CT) x-ray imaging, and nuclear imaging of particles emitted by radioactive tracers administered to a patient. For many cardiac, vascular, neurologic, and other imaging examinations using such modalities, synchronization of imaging data collection sequences to the intrinsic motion of the heart is crucial to obtain high-quality images without motion artifacts.
For cardiac synchronization of medical imaging data collection, triggering data collection by features recognized in the electrocardiogram (ECG) signal or in the peripheral pulse (PPU) signal from nearly any artery can be utilized. PPU signals, in comparison to ECG signals, are always delayed with respect to the QRS complex of the ECG signal (signifying the onset of ventricular systole), and have a temporal frequency spectrum with significantly lower frequency components. The variable delay can be up to 100-200 msec. or more. Therefore, triggering with ECG signals is generally preferred to achieve triggering that is temporally more precisely defined in time and to enable data collection during early ventricular systole.
However, important medical imaging modalities often introduce noise and distortions into ECG signals measured from a patient during imaging examinations. These noise and distortions are due to various kinds of electromagnetic interference inherently generated during examination by such modalities. In the case of CT and nuclear imaging, electrical interference may be induced in the patient or in affixed ECG leads, and therefore, into resulting ECG signals, either directly from the imaging apparatus itself or secondarily by radiation used in the imaging.
In the case of MR imaging, switched magnetic field gradients, RF pulses, and hydrodynamic flows of blood containing charged ions in the strong static magnetic field present in an MR apparatus induce voltage gradients in a patient that in turn introduce noise and distortions into ECG signals. Since blood flow effects are enhanced during systole, when the blood flows in the aorta and other vessels are increased, introduced ECG artifacts are maximum during ventricular systole. Such artifacts hamper R-wave detection and, thus, successful synchronization of imaging data collection.
For example, FIG. 2A illustrates two ECG signals from a healthy volunteer. ECG 80 is recorded in an MR apparatus but without the presence of the static main magnetic field. ECG 81 is recorded in the MR apparatus in the presence of a static field of, for example, 1.5 Tesla (T). Second ECG 81, recorded in the presence of the static magnetic field, contains several additional signal peaks compared to first ECG 80, which can lead to erroneous interpretations of the ECG, for example, being recognized as non-physiological, false R-waves.
A method and apparatus that attempts to improve ECG triggering is known from U.S. Pat. No. 5,526,813. In this known method, erroneous determination of ECG features is reduced by filtering and signal processing prior to using the signal for triggering imaging data collection. However, a drawback of this known method is that the filtering and signal processing of the ECG data can be complicated and unreliable, and as a result erroneous triggering of imaging data acquisition may still occur even though the ECG signals have been filtered and processed. Another method and apparatus that also attempts to improve ECG triggering is known from International Application no. PCT/IB98/01062. According to the latter known method, information from a vector ECG (VCG) signal is used to improve recognition of ECG signal features compared to their recognition in a scalar ECG. However, measurement of a vector electrocardiogram signal is also subject to inherent noise and distortions generated by the imaging modalities, and its analysis can also be complex, difficult, and unreliable.
What is needed, therefore, are simple and reliable methods and apparatus for accurately and reliably triggering data acquisition for medical image reconstruction at fixed times with respect to the cardiac cycle.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicants"" invention of the invention subsequently claimed.
The objects of the present invention are to provide methods and apparatus which overcome the above identified problems in, and satisfy the needs of, the current medical imaging art.
As used herein, it is to be understood that a xe2x80x9cmedical imaging modalityxe2x80x9d is any imaging modality that acquires imaging data by a process that can be disturbed by body motions, and, therefore, that advantageously takes heart motions into account when imaging organs that are directly or indirectly affected by such heart motions. This invention is most advantageously applied to those imaging modalities the practice of which generates noise and distortions of electrocardiogram (ECG) signals measured from a patient during imaging. Preferably, this invention is applied to magnetic resonance (MR) imaging, or to computer tomographic (CT) x-ray imaging, or to nuclear medicine imaging.
Additionally, as used herein, it is to be understood that xe2x80x9cECG signalsxe2x80x9d means any representation of the electrical activity of the heart. It includes conventional scalar representations where the time courses of single voltages measured between established positions on the patient are displayed. In these representations, the QRS complex has its well-known form, e.g., as schematically represented by signal 80 in FIG. 2A. It also includes vector electrocardiogram (VCG) representations, where the time course of the net electrical polarization vector of the heart is displayed in various projections. In these representations, the QRS complex is seen as a loop elongated along one direction. Finally, it is to be understood that xe2x80x9cPPU signalxe2x80x9d means any representation of the pulsatile flow of blood in an artery. It can be non-invasively measured, for example, by an oximeter, or invasively measured, for example, by arterial pressure, or by other means.
Generally, the objects of this invention are achieved by medical imaging methods according to which information from both ECG signals and from PPU signals are considered together in order to generate reliable synchronization signals for triggering imaging data collection in a medical imaging apparatus. The synchronization signals represent occurrences of pre-determined phases of cardiac motion so that the imaging data collection can be synchronized with the actual, physiologic, cyclic movements of the heart and with resulting blood flows. These objects are also achieved by medical imaging apparatus that, in addition to its conventional imaging means, includes further functional units necessary to practice the methods of medical imaging methods of this invention, such units as, for example, an ECG unit, and PPU unit and a synchronization unit.
Although, this invention is described herein primarily in its preferred embodiment directed to MR imaging and utilizing scalar ECG signal representations, it will be understood that this invention is not so limited. For example, it is equally applicable use of VCG signals, to CT imaging gated and to other imaging modalities. It is intended that these other embodiments apparent to one of skill in the art from the following drawing and description are also covered by the appended claims.
In detail, the objects of this invention are achieved by the following embodiments. In a first embodiment, the general methods of the invention include obtaining ECG signals representing the electrocardiogram of a patient placed in an examination zone of a medical imaging apparatus which collects medical imaging data for reconstruction of a medical image, obtaining PPU signals representing occurrences of peripheral pulses in the patient, providing one or more synchronization signals representing occurrences of one or more pre-determined phases of the cyclic movements of the heart, wherein the synchronization signals are provided in dependence on both the ECG signals and on the PPU signals, and controlling the medical imaging apparatus in dependence on the one or more synchronization signals in order to collect imaging data synchronized with cyclic movements of the heart from a part of the patient in the examination zone and to reconstruct a medical image of a part of the patient from the collected imaging data with reduced or absent motion artifacts.
In various aspects of the first embodiment, the part of the patient imaged includes cardiac structures, or intracranial structures, or vascular structures; the medical imaging apparatus is a magnetic resonance apparatus or a computed tomography x-ray apparatus; and the ECG signals include scalar ECG signals or vector ECG signals. In a further aspect, the synchronization signals are provided only if the PPU signals indicate that the pre-determined cardiac phases are physiologically possible. In this aspect, the general methods include determining, first, PPU-derived information from the PPU signals, wherein the PPU-derived information indicates time intervals within which the predetermined cardiac phases are physiologically more probable or less probable, and recognizing, second, the pre-determined cardiac phases in the ECG signal, wherein the recognizing is responsive to the probability or improbability of the pre-determined cardiac phases indicated by the PPU-derived information. Further, the PPU-derived information preferably indicates black-out intervals within which the pre-determined cardiac phases cannot occur, and within which the pre-determined cardiac phases are not recognized; or the PPU-derived information indicates window intervals only within which the predetermined cardiac phases can occur, and within which the pre-determined cardiac phases are recognized. Also, the provided synchronization signals preferably further include verification-type signals which indicate whether or not a previous synchronization signal represents a physiologic cardiac phase, and wherein the medical imaging apparatus is controlled to not use for image reconstruction imaging data collected in response to non-physiologic synchronization signals.
In further aspects of the first embodiment, the provided synchronization signals are R-wave-type synchronization signals that signal occurrences of R-waves in the ECG signals, and wherein the step of controlling collects imaging data in a pre-determined temporal relation to R-wave-type signals. Optionally, the pre-determined temporal relation is such that imaging data is collected during cardiac diastole. In one preferable alternative, from the PPU signal, black-out intervals are determined within which physiologic R-wave cannot occur in the ECG signals, and the R-wave-type synchronization signals are not provided during black-out intervals. Where the PPU signal comprises PPU complexes having positive lobes indicating peripheral systolic blood flow, the black-out intervals comprise the duration of the positive lobes of PPU complexes. In another preferable alternative, from the PPU signal, window intervals are determined within which physiologic R-waves in the ECG signal must occur, and the R-wave-type synchronization signals are provided only during window intervals. Where the PPU signal comprises PPU complexes having positive lobes indicating peripheral systolic blood flow, each window interval comprises an interval which begins at a pre-determined duration after the end of a just previous QRS complex in the ECG signal, and which ends at the beginning of the positive lobe of an immediately next PPU complex.
In a second embodiment, the invention includes a magnetic resonance apparatus for acquiring images of a part of a patient placed in an examination zone of the MR apparatus, the apparatus comprising a main magnet system for generating a steady magnetic field in the examination zone, a gradient magnet system for generating temporary gradient magnetic fields in the examination zone, a radio frequency (RF) transmitter system for generating RF pulses in the examination zone, an RF receiver system for receiving MR signals from the examination zone, an ECG system for obtaining ECG signals representing the electrocardiogram of the patient, a peripheral pulse unit system for obtaining PPU signals representing occurrences of peripheral pulses in the patient, a reconstruction unit for reconstructing an image of the region of the patient from the received MR signals, a control unit responsive to one or more synchronization signals for generating control signals controlling the gradient magnet system, the RF transmitter system, the RF receiver system, and the reconstruction unit, wherein the synchronization signals represent occurrences of one or more pre-determined phases of the cyclic movements of the heart, and wherein the control signals cause acquisition of MR data for the reconstruction an image of a part of the patient, and a synchronization unit for providing the one or more synchronization in dependence on both the ECG signals and on the PPU signals in order to synchronize the acquisition of MR signals with cyclic movements of the heart.
In various aspects of the second embodiment, the synchronization unit of the MR apparatus further comprises means for determining, first, PPU-derived information from the PPU signals, wherein the PPU-derived information indicates time intervals within which the pre-determined cardiac phases are physiologically more probable or less probable, and means for recognizing, second, the pre-determined cardiac phases in the ECG signals, wherein the recognizing is responsive to the probability or improbability of the pre-determined cardiac phase indicated by the PPU-derived information. Preferably, the synchronization unit further comprises one or more programmable elements, and one or more memories for storing instructions for causing the synchronization to function for providing the synchronization signals in dependence on both the ECG signals and on the PPU signals.
In a third embodiment the invention includes a computed tomography (CT) x-ray apparatus for acquiring images of a part of a patient placed in an examination zone of the CT apparatus, the apparatus comprising a radiation source, a detector unit which is coupled to the radiation source, means for causing the radiation source and the detector unit to perform a rotational scanning motion about the patient in the examination zone during which scanning motion measuring data is acquired, an ECG system for obtaining ECG signals representing the electrocardiogram of the patient, a peripheral pulse unit system for obtaining PPU signals representing occurrences of peripheral pulses in the patient, a reconstruction unit for reconstructing the spatial distribution of the absorption within the patient from the measuring data acquired by the detector unit, and a control unit responsive to one or more synchronization signals for generating control signals controlling the radiation source, the detector unit, the means for causing a rotational scan, and the reconstruction unit, wherein the synchronization signals represent occurrences of one or more pre-determined phases of the cyclic movements of the heart, and wherein the control signals cause acquisition of measuring data for the reconstruction an image of a part of the patient, and a synchronization unit for providing the one or more synchronization signals in dependence on both the ECG signals and on the PPU signals in order to synchronize the acquisition of measuring data with cyclic movements of the heart.
In a fourth embodiment, the invention includes a computer readable media carrying encoded program instructions for causing a medical imaging apparatus to perform the method of claim 1.